This allows us to remove the dependency on the XML serializer provided
by the external `activemodel-serializers-xml` gem, and eliminates the
following deprecation warning:
DEPRECATION WARNING: ActiveModel::Errors#to_xml is deprecated and
will be removed in Rails 6.2.
Please note: this does not mean Devise doesn't support XML, it simply
means our test suite will use JSON to test non-navigatable formats
instead of XML, for simplicity. Devise's job is not to test object
serialization, so as long as your objects properly serialize to
XML/JSON/any other format, it should work out of the box.
Peter Goldstein